Ischemic heart disease and occlusive coronary artery disease continue to be the number one killer in the developed world. There are an estimated 500,000 acute ST-elevation myocardial infarctions (MI) in the US alone each year [1], and this is becoming an increasingly significant problem in the developing world [2]. Current approaches for management of an acutely occluded coronary artery leading to an MI consist of anti-platelet and anti-thrombotic strategies with intervention aimed at opening the infarct-related artery in a timely fashion. Although this approach is able to protect cardiomyocytes from necrosis, with resulting decrease in morbidity and mortality, it necessitates exposing the heart to post-ischemic reperfusion injury. Limiting this reperfusion injury and decreasing apoptosis would ultimately lead to greater myocardial salvage and prevention of development of heart failure. Numerous animal studies have identified biological agents able to ameliorate this ischemia-reperfusion injury and reduce the ultimate infarct size [3, 4]. However, further development of these approaches is hindered by the inability to deliver the biologic agents to the myocardium in a tissue-specific, efficient and rapid manner. A protein transduction peptide specific for the heart would be able to deliver biologic agents in a timely fashion to the heart when given at the time of reperfusion for an infarction:
Protein transduction domains (PTD) are small cationic peptides that can cross cellular membranes, and are able to transport large, biologically active molecules into mammalian cells in culture as well as in vivo. The limitation of PTDs is the non-specific transduction of all tissue types with some tissues, such as liver and kidney, taking up the PTD much more avidly than heart tissue. Thus there is a need to identify peptides able to target cardiac tissue specifically for delivery of biologics of therapeutic potential.
Screening approaches using peptide phage display libraries are effective for identifying peptides able to bind to specific ligand targets as well as identifying peptides with novel properties. Phage display uses filamentous bacteriophage, such as M13, that are able to replicate in E. coli. The proteins or peptides to be displayed are fused to the N-terminus of phage coat protein pIII or pVIII and thus are present on the surface of the phage. Screening of peptide phage display libraries has been used in vivo to identify peptides able to target tumor vasculature [5], adipose tissue [6] and pancreatic islet cells [7]. In addition, it has been used to identify peptides able to facilitate internalization of intact, infectious phage into specific cell types such as synovial fibroblasts [8]. In vivo phage display also has been utilized to target atherosclerotic plaques [9], and to probe the heart vasculature for endothelial markers [10]. Although in vitro selection of a specific peptide sequence carrying phage resulted in increased targeting of cardiomyocytes by phage in vivo [11], it remains to be determined if the peptide can actually deliver “cargo” peptides or proteins of therapeutic potential to the heart. If such were indeed the case, it would open up new avenues of drug development, leading to delivery of therapeutics directly to the ischemic heart.